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A five star review

When the Government set up the National Flood Resilience Review, the Met Office played a central role.

Following the extreme rainfall and flooding in Cumbria in December 2015, the Government set up the National Flood Resilience Review (NFRR) to assess the risks of flooding across England.

The review quickly became a priority after serious flooding hit York on Boxing Day. The NFRR modelling was led by the Environment Agency with the Met Office contributing weather and climate expertise. Our contribution was led by Ian Lisk, Head of Environmental Hazards and Partnerships at the Met Office.

“The Met Office was asked to generate a set of plausible extreme rainfall scenarios based on innovative use of the latest advances in our weather and climate modelling science,” says Ian.

The Met Office also looked at whether the recent run of extreme rainfall events reflects an underlying trend in the frequency of extreme weather. While there is evidence that climate change is affecting rainfall over the country, for the review we concluded that variability due to natural processes will continue to dominate in terms of extreme rainfall over the next ten years. “Our plausible extreme rainfall scenarios were run through the river models of the six catchments chosen by the Environment Agency as a stress test of their existing flood risk assessments,” explains Ian.

The process used in the review will also help to inform future flood risk assessment strategies. “The intention now is to use the techniques we’ve developed more widely to help inform the next National Flood Risk Assessment in 2018,” says Ian.

“Beyond that, in line with NFRR recommendations, we’ll be looking to work with partners on better linking up meteorology, hydrology and flood modelling science through the development of a more integrated flood risk modelling approach,” he added.

Leading the way in data

We were asked to develop extreme rainfall scenarios that were scientifically valid and plausible. Our novel and innovative approach taken was endorsed by the NFRR’s Scientific Advisory Group and corroborated by results from the European Centre for Medium Range Weather Forecasting.

The state-of-the-art Met Office Hadley Centre climate model produced over 11,000 monthly rainfall scenarios for six large regions in England and Wales and for the current climate. These were used to sample many more cases than are available from existing observational records, including several hundred extreme regional rainfall events that are meteorologically possible but lie outside what has been experienced based on our observational records.

The chance of extreme events like these happening was then estimated. The results suggest there is a 1% likelihood every year that winter monthly rainfall totals could plausibly be 20% higher than recent past extremes in some parts of the country and in other areas up to 30% higher than recent past extremes. Over any of the large regions there is also around a 10% chance in any given year of existing monthly rainfall records being matched or broken.